1. Field of the Invention:
The present invention relates to a receiving circuit suitable for use in a mobile communication apparatus, and particularly to a receiving circuit using CDMA (Code Division Multiple Access).
2. Description of the Related Art:
CDMA is a technique used in digital communications, particularly, mobile communications typified by a cellular telephone. CDMA, particularly, DS-CDMA (Direct Sequence CDMA) makes it possible to bring an information signal into a broad band by use of a spreading code independent of an information signal bandwidth and use a pseudo noise sequence or an orthogonal code or the like for the spreading code to thereby simultaneously use the same carrier frequency in several information signal communications.
A receiving circuit of the DS-CDMA system de-spreads a receive signal using a spreading code used in a desired information signal in order to take out or extract an information signal.
On the other hand, when it is desired to implement a DS-CDMA type receiver according to digital signal processing, it needs an A/D converter.
In the conventional voice communications or the like in which the amount of information per unit time, to be transmitted/processed is low, a sampling rate of an A/D converter has been set to eight times the spreading band.
However, in recent voice communications employed in multimedia communications or the like such as motion pictures, etc., the amount of information per unit time increases and a spreading bandwidth expands spontaneously. Therefore, when the eight times the spreading bandwidth is taken as the sampling rate as in the case of the conventional A/D converter, this causes an increase in power requirements of a receiving circuit and leads to an increase in the cost of the receiving circuit.
However, the mere reduction in the sampling rate of the A/D converter would incur degradation in receiving performance evaluated by a receiving BER (Bit Error Rate) or the like.
Thus, the purpose of the present invention is to provide a receiving circuit which does not cause degradation in receiving performance evaluated by a receiving BER or the like even if a sampling rate of an A/D converter is reduced.